1. Field of the Invention
The present invention relates to a thermally activating apparatus for a heat-sensitive adhesive sheet, for example, used as an adhesive label, having a heat-sensitive adhesive layer that exhibits a non-bonding property normally and expresses a bonding property by heat on one side of a sheet-like substrate, and to a printer using the thermally activating apparatus, and more particularly, relates to a technique effective in application for energy control when the heat-sensitive adhesive layer is thermally activated.
2. Description of the Related Art
Many labels adhered to commodities and used for bar-code or price indication have recently had a pressure-sensitive adhesive layer on the back of a recording surface (printing surface), on which a released paper (separator) was bonded, and were stored in a temporarily bonded state. However, when these types of adhesive labels are used as labels, the released paper must be released from the pressure-sensitive adhesive layer, thus having a problem in that refuse is inevitably produced.
Accordingly, a heat-sensitive adhesive label and a thermally activating apparatus have been developed as a system that requires no released paper, the adhesive label having on the back of a label-like substrate a heat-sensitive adhesive layer that exhibits a non-bonding property normally but expresses a bonding property by heat, and the thermally activating apparatus heating the heat-sensitive adhesive layer on the back of the label to make it exhibit a bonding property.
Various types of heating systems have been proposed for the thermally activating apparatus, which employ a heating roll system, a hot-air blowing system, an infrared-ray radiating system, and a system using an electric heater or a dielectric coil as a heating means. For example, Japanese Unexamined Patent Application Publication No. 11-79152 discloses a technique in which a head having one or a plurality of resistance elements (heating devices) provided on a ceramic substrate as a heat source, such as a thermal head used as a printing head of a thermal printer, is brought into contact with a heat-sensitive adhesive label to heat a heat-sensitive adhesive layer.
The thermally activating apparatus for the heat-sensitive adhesive layer that is disclosed in the Japanese Unexamined Patent Application Publication No. 11-79152 is composed of a thermally activating platen roller serving as a transfer means for carrying a heat-sensitive adhesive label and a thermally activating thermal head having a heating device serving as a heating means. The heating device is formed of a heating resistance element formed on a ceramic substrate, on which a protective film made of glass ceramics is formed so as to cover the surface of the heating resistance element. The thermally activating platen roller functions also as a pressurizer for sandwiching the heat-sensitive adhesive label between it and the heating device.
According to the aforesaid prior art, since the heating device is heated by energizing the heating device in a state in which the thermal head serving as a heating means is in contact with the heat-sensitive adhesive layer, the heat-sensitive adhesive layer is thermally activated reliably; moreover, since the heat from the heating device can efficiently be conducted to the heat-sensitive adhesive layer, there is an advantage of requiring less power consumption.
In the thermally activating apparatus that uses the aforesaid thermal head as a heating means, generally, energy is applied to each heating device by the energization/break of one pulse to activate the adhesive of the heat-sensitive adhesive sheet. In this case, since the thermal head is subjected to relatively high energy at a time because it includes a plurality of heating devices, the caloric value of the thermal head is increased to increase the ultimate temperature of the surface inevitably. Accordingly, the surface temperature of the thermal head becomes higher than the carbonizing temperature of a resin component of the adhesive; thus, the resin component is sometimes carbonized and fixed to the surface of the thermal head. On the other hand, when the amount of energy applied with one pulse is set small so that the surface temperature of the thermal head does not exceed the resin carbonizing temperature, the adhesive cannot sufficiently be activated, thus posing a problem in that the bonding property is decreased.
FIG. 7 shows an energy control method in the conventional thermally activating apparatus, showing the relationship between an energized pulse (b) and surface temperature (a) of the thermal head. A case of repeating an operation of carrying a voltage of 24 V for 1 ms and breaking it for 2 ms is shown as an example. By the method of FIG. 7, a portion of the heat-sensitive adhesive sheet, which is in contact with the thermal head, is thermally activated by passing one pulsed electricity to transfer the heat-sensitive adhesive sheet, and the whole surface of the heat-sensitive adhesive sheet is thermally activated by sequentially passing pulsed electricity. Here, since the amount of heat (energy) generated from the heating device (resistance element) of the thermal head is proportional to the second power of the carried voltage and time, the diagonally shaded areas of FIG. 7(b) correspond to energy that is transmitted from the heating device to the heat-sensitive adhesive.
As shown in FIG. 7, when the energy necessary for activating the heat-sensitive adhesive is applied with one pulse, the heating device continues to generate heat for 1 ms, thus suddenly increasing the surface temperature of the thermal head. Therefore, the surface temperature of the thermal head sometimes reaches 300° C. although a heat-sensitive adhesive having a resin carbonizing temperature of, for example, 250° C. is used.
As described above, according to the conventional energy control method, the surface temperature of the thermal head exceeds the resin carbonizing temperature of the heat-sensitive adhesive, therefore posing a problem of carbonizing and fixing a resin component. In other words since the carbide of the resin component prevents heat transfer from the thermal head to the heat-sensitive adhesive, energy transfer efficiency is decreased, thus producing a problem of not exhibiting the bonding property of the heat-sensitive adhesive sufficiently.
Since the optimum energy for thermal activation differs depending on the type of the heat-sensitive adhesive and ambient temperature, there is a problem in that it is difficult to exhibit a desired bonding property.
FIG. 8 shows the relationship between a bonding property that is exhibited, for example, when two types of adhesives A and B are subjected to a thermal energy of 0.6 mJ, and ambient temperature. The adhesive A (shown by a solid line in the drawing) is of low-temperature bonding type which is easily thermally activated in a relatively low temperature range (for example, to 10° C.), and the adhesive B (shown by a dashed and dotted line in the drawing) is of a normal-temperature type which is easily thermally activated in a normal temperature range (for example, 15° C. to 25° C.).
For example, when an energy of 0.6 mJ is applied to such two types of adhesives at an ambient temperature in the vicinity of TA for thermal activation, the adhesive A can exhibit a predetermined bonding property F or more; however, the adhesive B exhibits a bonding property of F or less. In other words, when the adhesive B is thermally activated at an ambient temperature in the vicinity of TA, it is necessary to apply more energy (for example, to increase energizing time).
However, in the conventional thermally activating apparatus, since the applied energy is not strictly controlled depending on the type of adhesive and ambient temperature, a desired bonding property could not be exhibited.